Group III nitride compound semiconductor device

ABSTRACT

Aluminum gallium nitride (Al x Ga 1−x N, 0&lt;x&lt;1) is employed as a substrate of a Group III nitride compound semiconductor device. In light-emitting diodes and laser diodes employing the substrate, crack generation is prevented, even when a thick cladding layer formed of aluminum gallium nitride (Al x Ga 1−x N, 0&lt;x&lt;1) is stacked on the substrate. The smaller the difference in Al compositional proportion between the substrate and an aluminum gallium nitride (Al x Ga 1−x N, 0&lt;x&lt;1) layer, the less likely the occurrence of crack generation.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a Group III nitride compoundsemiconductor device and, more particularly, to a Group III nitridecompound semiconductor device which functions as a light-emittingdevice.

[0003] 2. Background Art

[0004] A Group III nitride compound semiconductor is one type ofsemiconductors used among direct transition type of semiconductors,which have an emission spectrum widely ranging from ultraviolet to red.Thus, the semiconductor is employed in light-emitting devices such aslight-emitting diodes (LEDs) or laser diodes (LDs). In a devicefabricated by stacking Group III nitride compound semiconductor layers,a sapphire substrate is typically employed as a substrate in view(because) of its proximity in lattice constant. FIGS. 3 and 4 showstructures of the semiconductor devices.

[0005] Since sapphire is an electric insulator, stacked Group IIInitride compound semiconductor layers are partially etched to form ann-type contact layer to thereby form a negative electrode. Then, apositive electrode is formed on a non-etched uppermost layer of thesemiconductor. FIG. 3 shows a conventional light-emitting diode (LED)900 employing Group III compound semiconductors. In the LED, an AlNbuffer layer 902, an n-GaN n-contact layer 903, an n−Al_(x)Ga_(1−x)Nn-cladding layer 904, an active layer (emission layer) 905 formed of asingle layer or a preferably a multi-layer (single quantum well ormultiple quantum well), a p-Al_(x)Ga_(1−x)N p-cladding layer 906, and ap-GaN p-contact layer 907 are formed on a sapphire substrate 901, in theorder presented. A positive electrode 908A is formed on the p-contactlayer 907 while a negative electrode 908B, developed through etching, isformed on a portion of the n-contact layer 903.

[0006]FIG. 4 shows a conventional laser diode (LD) 950 employing GroupIII compound semiconductors. In the LD, an AlN buffer layer 912, ann-GaN n-contact layer 913, an n-Al_(x)Ga_(1−x)N n-cladding layer 914, ann-GaN n-guide layer 915, an emission layer 916 formed, preferably of amulti-layer (multiple quantum well, MQW), a p-GaN p-guide layer 917, ap-Al_(x)Ga_(1−x)N p-cladding layer 918, and a p-GaN p-contact layer 919are formed on a sapphire substrate 911, in the order presented. Apositive electrode 920A is formed on the p-contact layer 919 while anegative electrode 920B, developed through etching, is formed on aportion of the n-contact layer 913.

[0007] However, the aforementioned conventional semiconductor has adrawback. Specifically, when a Group III nitride compound semiconductoris formed on a sapphire substrate, cracks are generated in asemiconductor layer, or a semiconductor layer bends, since elasticmodulus and thermal expansion coefficient of the sapphire substratediffer from those of the Group III nitride compound semiconductor. Thus,the fabricated device has poor device characteristics. In addition,although lattice constant of the sapphire substrate is approximatelyequal to that of the Group III nitride compound semiconductor,dislocations are readily generated due to misfit. Particularly, acladding layer formed of Al_(x)Ga_(1−x)N attains a higher elasticmodulus as the compositional proportion of Al (hereinafter simplyreferred to as “x”) increases. Therefore, cracks are readily generatedin such a cladding layer during a cooling process in production of asemiconductor device. As a result, the thickness of the cladding layer,which has a large compositional proportion of Al is limited to a lowvalue. Such limitation in thickness is particularly detrimental tofabrication of laser diodes.

[0008] Employment of a sapphire substrate, which is an electricinsulator, raises another limitation for fabricating semiconductordevices. Specifically, a positive electrode and a negative electrodemust be disposed on a semiconductor-formed surface of a sapphiresubstrate.

SUMMARY OF THE INVENTION

[0009] In view of the foregoing disadvantages, an object of the presentinvention is to provide a Group III nitride compound semiconductordevice in which generation of cracks in a semiconductor layer, bendingof a semiconductor layer, and generation of misfit-induced dislocationin a semiconductor layer are prevented. Another object of the inventionis to provide a Group III nitride compound semiconductor deviceemploying a conductive substrate through which electricity is passed.

[0010] Accordingly, the present invention is directed to a Group IIInitride compound semiconductor device comprising a substrate and one ormore Group III nitride compound semiconductor layers formed on a firstsurface or first and second surface of the substrate, wherein aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) is employed as the substrate.

[0011] Preferably, among the Group III nitride compound semiconductorlayers stacked on a first surface or first and second surfaces of asubstrate, all layers, having a thickness of more than 10 nm, areindependently formed of a compound represented by Al_(x)Ga_(1−x)N,wherein 0≦x≦1.

[0012] Preferably, each of the Group III nitride compound semiconductorlayers stacked on a first surface or first and second surfaces of asubstrate is formed of a compound represented by Al_(x)Ga_(1−x)N,wherein 0≦x≦1.

[0013] Preferably, a first layer of a Group III nitride compoundsemiconductor layer stacked on a first surface or a first and secondsurface of a substrate has a thickness of 1-20μm, more preferably 2-20μm.

[0014] A group III nitride compound semiconductor layer comprises binarycompounds such as AlN, GaN, and InN. A group III nitride compoundsemiconductor layer also comprises ternary compounds such asAl_(x)Ga_(1−x)N, Al_(x)In_(1−x)N, and Ga_(x)In_(1−x)N (0<x<1). And agroup III nitride compound semiconductor layer further comprisesquaternary compounds such as Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1,0≦x+y1). In the present invention, unless otherwise specified, the term“Group III nitride compound semiconductors” encompasses Group IIInitride compound semiconductors per se and Group III nitride compoundsemiconductors doped with an impurity which causes the semiconductors tobecome either p- or n-conduction type semiconductors. Likewise, aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) also encompasses dopedsemiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Various other objects, features, and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood with reference to the following detaileddescription of the preferred embodiments when considered in connectionwith accompanying drawings, in which:

[0016]FIG. 1 is a schematic cross-sectional view of the structure of aGroup III nitride compound semiconductor device according to a firstembodiment of the present invention;

[0017]FIG. 2 is a schematic cross-sectional view of the structure of aGroup III nitride compound semiconductor device according to a secondembodiment of the present invention;

[0018]FIG. 3 is a schematic cross-sectional view of the structure of aconventional light-emitting diode comprising a Group III nitridecompound semiconductor; and

[0019]FIG. 4 is a schematic cross-sectional view of the structure of aconventional laser diode comprising a Group III nitride compoundsemiconductor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] A Group III nitride compound semiconductor device such as alight-emitting device is produced by doping an appropriate impurity to aGroup III nitride semiconductor component layer represented byAl_(x)Ga_(y)In_(1−x−y)N (0≦x≦1; 0≦y≦1; 0≦x+y≦1). As discussed above, thesemiconductor layers, cracs are readily generated in an aluminum galliumnitride (Al_(x)Ga_(1−x)N, 0<x<1) layer when the layer is formed so as tohave a large thickness, since the sapphire substrate has an elasticmodulus and thermal expansion coefficient greatly different from thoseof the aluminum gallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) layer. Thus,when a substrate in which ‘x’ is equal or nearly equal to the ‘x’ in analuminum gallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) layer is formed in thedevice, the difference in elastic modulus and thermal expansioncoefficient between the substrate and the semiconductor layer can besuppressed to a minimum level. Accordingly, even when a substrate has athickness of 50 μm or more, further 100 μm or more, or an aluminumgallium nitride (Al_(x)Ga_(1−x)N, 0<x<1) cladding layer is formed so asto have a thickness of 1 μm or more, no cracks are generated, due to thesmall difference in the elastic modulus and thermal expansioncoefficient between the substrate and a Group III nitride semiconductorcomponent layer. The cladding layer preferably has a thickness of 1 μmor more in order to ensure the crystallinity of a layer formed thereon.In connection with the upper limit, the thickness is preferably 20 μm orless, more preferably 2-10 μm, in view of the productivity of devices.

[0021] When each Group III nitride compound semiconductor layer isformed from Al_(x)Ga_(1−x)N (0≦x≦1); such as, gallium nitride, aluminumgallium nitride, or aluminum nitride, the overall thickness of thestacked layers is approximately 20 μm. In such a semiconductor, theformation of cracks, induced by the difference in the elastic modulusand thermal expansion coefficient between the substrate and a Group IIInitride semiconductor component layer can be prevented. When an emissionlayer of a light-emitting device is formed of a multiple quantum well, awell layer and a barrier layer, the emission layer may be sufficientlythin and does not have to be formed of Al_(x)Ga_(1−x)N (0<x<1). Such amode is also included within the scope of the invention. Theaforementioned Group III nitride compound semiconductor devices areparticularly preferred as light-emitting devices, such as light-emittingdiodes (LEDs) and laser diodes (LDs), which require a long service life.Specifically, in the production of an LD, when a thick cladding layer isformed from an aluminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1) and,analuminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1) in which ‘x’ is equalor nearly equal to ‘x’ in the cladding layer is used, the formation ofcracks, induced by the difference in the elastic modulus and thermalexpansion coefficient between the substrate and a Group III nitridesemiconductor component layer can be prevented. When the ‘x’ of thecladding layer coincides with the ‘x’ of the substrate, the ‘x’ can beelevated and all semiconductor component layers can contain aluminum.Furthermore, when aluminum gallium nitride Al_(x)Ga_(1−x)N (0<x<1)having a desired electric conductivity is employed as a substrate, aGroup III nitride compound semiconductor light-emitting device isproduced in which electricity is passed via the aluminum gallium nitridesubstrate.

[0022] The Group III nitride compound semiconductor device of thepresent invention may also be adapted to power devices andlight-emitting devices other than LEDs and LDs. When a contact electrodeis formed on a substrate, a Si-doped GaN contact layer may be formed soas to lower the contact resistance. Semiconductor component layers mayalso be formed of a composition-graded layer; e.g., from GaN toAl_(x)Ga_(1−x)N.

[0023] The present invention will next be described in detail withreference to specific embodiments, which should not be construed aslimiting the invention thereto.

[0024] First Embodiment

[0025] A light-emitting diode 100 having a structure shown in FIG. 1 wasproduced. The LED 100 contains an n-type A1 _(0.07)Ga_(0.93)N substrate1 having a thickness of approximately 100 μm and an electron density of3×10¹⁷/cm³.

[0026] The following layers were formed on the n-typeAl_(0.07)Ga_(0.93)N substrate 1, in the order presented, an Si-dopedAl_(0.07)Ga_(0.93)N n-type cladding layer 2 having a thickness ofapproximately 0.5 μm and an electron density of 2×10¹⁸/cm³; a GaN activelayer 3 having a thickness of 50 nm; an Mg-doped Al_(0.07)Ga_(0.93)Np-type cladding layer 4 having a thickness of approximately 0.5 μm and ahole density of 5×10¹⁷/cm³. A metal electrode 5A was formed on thecladding layer 4, and a metal electrode 5B was formed on the entirebackside of the n-type Al_(0.07)Ga_(0.93)N substrate 1.

[0027] The metal electrode 5A, serving as a positive electrode, wasformed from gold (Au). Alternatively, the electrode may be formed fromAu—Co alloy, Au—Ni alloy, Au—other metal alloy, or a multi-layerthereof. The metal electrode 5B, serving as a negative electrode, wasformed from aluminum (Al). Alternatively, the electrode may be formedfrom Al—V alloy, Al—Ti alloy, Al— other metal alloy, or a multi-layerthereof.

[0028] A method for producing the light-emitting diode 100 shown in FIG.1 will next be described. In practice, the Al_(0.07)Ga_(0.93)N substrate1 was produced by epitaxially growing Al_(0.07)Ga_(0.93)N on a silicon(Si) substrate employing a halide source and removing the silicon Sisubstrate. More specifically, a silicon (Si),substrate was placed in achamber equipped with a halogen source-supplier. The chamber wasevacuated, and nitrogen (N₂) was fed into the chamber. The atmospherewas heated to 1000° C., to thereby cause gallium (Ga) and aluminum (Al)to react with hydrogen chloride (HCl). The chloride formed was suppliedto the Si substrate, while ammonia (NH₃) was introduced to the chamber,thereby forming Al_(0.07)Ga_(0.93)N on the Si substrate.

[0029] The Si substrate was removed through mechanical polishing usingdiamond particles and mechanochemical polishing that employs colloidalsilica particles in an alkaline medium, to thereby obtain the n-typeAl_(0.07)Ga_(0.93)N substrate 1 having an electron density of 3×10¹⁷/cm³and a thickness of 100 μm. Alternatively, the n-type Al_(0.07)Ga_(0.93)Nsubstrate 1 may be obtained by removing an Si substrate throughwet-etching.

[0030] A method for producing the component layers of the light-emittingdiode 100 of the invention will next be described. The light-emittingdiode 100 was produced through a vapor phase growth method such as,metal organic vapor phase epitaxy (MOVPE). Employed-gases were ammonia(NH₃), carrier gases (N₂ or H₂), trimethylgallium (Ga(CH₃)₃, hereinafterabbreviated as TMG), trimethylaluminum (Al(CH₃)₃, hereinafterabbreviated as TMA), trimethylindium (In(CH₃)₃, hereinafter abbreviatedas TMI), silane (SiH₄), and cyclopentadienylmagnesium (Mg(C₅H₅)₂,hereinafter abbreviated as CP₂Mg).

[0031] Initially, an n-type Al_(0.07)Ga_(0.93)N substrate 1 was placedon a susceptor disposed in a reaction chamber of an MOVPE apparatus, andthe temperature of the substrate was maintained at 1000° C. A carriergas (10 L/min), ammonia (NH₃) (10 L/min), TMG (100 μmol/min), TMA (5μmol/min), and silane (SiH₄, diluted to 0.86 ppm with hydrogen) (5nmol/min) were fed to the chamber, to thereby form an Si-dopedAl_(0.07)Ga_(0.93)N n-type cladding layer 2 having a thickness ofapproximately 0.5 μm and an electron density of 2×10¹⁸/cm³.Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10 L/min), andTMG (20 μmol/min) were fed to the chamber, to thereby form a GaN activelayer 3. Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10L/min), TMG (100 μmol/min), TMA (5 μmol/min), and CP₂Mg (2 μmol/min)were fed into the chamber, to thereby form an Mg-dopedAl_(0.07)Ga_(0.93)N p-type cladding layer 4 having a thickness ofapproximately 0.5 μm. The cladding layer 4 was irradiated with anelectron beam so as to lower electric resistance of the layer, therebycausing a p-type cladding layer 4 to have a hole density of 5×10¹⁷/cm³.Gold (Au) was vapor-deposited on the p-type cladding layer 4 to form apositive electrode, and aluminum (Al) was vapor-deposited on thebackside of the n-type Al_(0.07)Ga_(0.93)N substrate 1 to form anegative electrode.

[0032] The thus-produced light-emitting diode 100 exhibited an emissionpeak wavelength of 365 nm.

[0033] The light-emitting diode 100 shown in FIG. 1 has the emissionlayer sandwiched between the positive electrode 5A and the negativeelectrode 5B. Comparing FIGS. 1 and 3, an etching step for exposing then-type contact layer 903 included in the conventional light-emittingdiode 900 can be omitted. As a result, the number of layers to be formedcan be reduced. In addition, the metal electrode contact area (ohmiccontact area) of the positive and negative electrodes can be enhanced.

[0034] Second Embodiment

[0035] A laser diode 200 having a structure shown in FIG. 2 wasproduced. The LD 200 contains an n-type Al_(0.07)Ga_(0.93)N substrate 11having a thickness of approximately 100 μm and an electron density of3×10¹⁷/cm³.

[0036] On the n-type Al_(0.07)Ga_(0.93)N substrate 11 were formed, inthe order given, an Si-doped A1 _(0.07)Ga_(0.93)N n-type cladding layer12 having a thickness of approximately 3 μm and an electron density of2×10¹⁸/cm³; an Si-doped Al_(0.01)Ga_(0.99)N n-type guide layer 13 havinga thickness of approximately 0.5 μm and an electron density of5×10¹⁷/cm³; an emission layer 14 of a multiple quantum well (MQW)structure comprising five GaN well layers each having a thickness of 2nm stacked alternately with six Al_(0.01)Ga_(0.99)N barrier layers eachhaving a thickness of 5 nm; an Mg-doped Al_(0.01)Ga_(0.99)N p-type guidelayer 15 having a thickness of approximately 0.5 μm and a hole densityof 5×10¹⁷/cm³; an Mg-doped Al_(0.07)Ga_(0.93)N p-type cladding layer 16having a thickness of approximately 1 μm and a hole density of5×10¹⁷/cm³; and an Mg-doped GaN p-type contact layer 17 having athickness of approximately 0.2 μm and a hole density of 7×10¹⁷/cm³. Ametal electrode 18A was formed on the p-type contact layer 17, and ametal electrode 18B was formed on the entire backside of the n-type A1_(0.07)Ga_(0.93)N substrate 11. The laser diode 200 of the secondembodiment was produced, on the n-type n-type Al_(0.07)Ga_(0.93)Nsubstrate 11 through a metal organic vapor phase epitaxy (MOVPE) method,in a manner similar to that described in the first embodiment.Specifically, an n-type Al_(0.07)Ga_(0.93)N substrate 11 was placed on asusceptor disposed in a reaction chamber of an MOVPE apparatus, and thetemperature of the substrate was maintained at 1000° C. A carrier gas(10 L/min), ammonia (NH₃) (10 L/min), TMG (100 μmol/min), TMA (5μmol/min), and silane (SiH₄, diluted to 0.86 ppm with hydrogen) (5nmol/min) were fed into the chamber, to thereby form an Si-dopedAl_(0.07)Ga_(0.93)N n-type cladding layer 12 having a thickness ofapproximately 3 μm and an electron density of 2×10¹⁸/cm³. Subsequently,a carrier gas (10 L/min), ammonia (NH₃) (10 L/min), TMG (50 μmol/min),and TMA (1 μmol/min) were fed to the chamber, to thereby form anSi-doped Al_(0.01)Ga_(0.93)N n-type guide layer 13 having, a thicknessof 0.5 μm and an electron density of 5×10¹⁷/cm³. Subsequently, theemission layer 14 of a multiple quantum well (MQW) structure was formed.The MQW layer comprises five GaN well layers stacked alternately withsix Al_(0.01)Ga_(0.99)N barrier layers. Specifically, GaN well layerseach having a thickness of 2 nm are formed by feeding ammonia (NH₃),TMG, and Al_(0.01)Ga_(0.99)N barrier layers each having a thickness of 5nm were formed by feeding ammonia (NH₃), TMG, and TMA.

[0037] Subsequently, a carrier gas (10 L/min), ammonia (NH₃) (10 L/min),TMG (50 μmol/min), TMA (1 μmol/min), and CP₂Mg (2 μmol/min) were fed tothe chamber, to thereby form an Mg-doped Al_(0.01)Ga_(0.93)N p-typeguide layer 15 having a thickness of approximately 0.5 μm. Subsequently,a carrier gas (10 L/min), ammonia (NH₃) (10 L/min), TMG (50 μmol/min),TMA (5 μmol/min), and CP₂Mg (2 μmol/min) were fed to the chamber, tothereby form an Mg-doped Al_(0.07)Ga_(0.93)N p-type cladding layer 16having a thickness of approximately 1 μm. Subsequently, a carrier gas(10 L/min), ammonia (NH₃) (10 L/min), TMG (100 μmol/min), and CP₂Mg (2μmol/min) were fed to the chamber, to thereby form an Mg-doped GaNp-type contact layer 17 having a thickness of approximately 0.2 μm. Thep-type contact layer 17, the p-type cladding layer 16, and the p-typeguide layer were irradiated with an electron beam so as to lowerelectric resistance thereof, thereby causing the layers to have holedensities of 7×10¹⁷/cm³, 5×10¹⁷/cm³, and 5×10¹⁷/cm³, respectively. Gold(Au) was vapor-deposited on the p-type contact layer 17 to form apositive electrode 18A, and aluminum (Al) was vapor-deposited on thebackside of the n-type Al_(0.07)Ga_(0.93)N substrate 11 to form anegative electrode 18B.

[0038] In contrast to a conventional laser diode such as the laser diode950 shown in FIG. 4, cracks were not generated in the thus-producedlaser diode 200, despite the n-type cladding layer 12 formed thereinhaving a thickness as large as approximately 3 μm. The threshold currentof the laser diode 200 of the present invention is 100 mA, whereas theconventional laser diode 950 has a threshold current of 200 mA. Thereason for the decrease in threshold current is thought to beenhancement in the light confinement due to an absence of cracks. In thelaser diode 200, facets having a mirror surface can readily be attainedthrough cleavage. In addition, electric current can be caused to flowfrom the p-type GaN contact layer 17 to the substrate 11, to therebyomit an n-type GaN contact layer and enhance the metal electrode contactarea (ohmic contact area) of the negative electrode.

[0039] In the above-described both embodiments, a silicon substrate wasemployed for forming the Al_(0.07)Ga_(0.93)N substrate through halideepitaxy. However, other substrates, such as silicon carbide (SiC),gallium phosphide (GaP), and sapphire, may also be employed. Although inthe above-described both embodiments, Group III nitride compoundsemiconductor layers are formed on the Al_(0.07)Ga_(0.93)N substratethrough metal organic chemical vapor deposition (MOCVD), other vaporphase growth methods, such as molecular-beam epitaxy (MBE), halide vaporphase epitaxy (Halide VPE), and liquid-phase epitaxy (LPE) may also beemployed.

[0040] The structure of the light-emitting device is not particularlylimited, and the device may have a homo-,hetero-, ordouble-hetero-structure. These structures may be formed via an MISjunction, a PIN junction, or a pn junction. The emission (active) layermay have any quantum well structure, for example, a single quantum well(SQW) structure, or a multiple quantum well (MQW) structure whichcomprises well layers and barrier layers having a band gap higher thanthat of a well layer.

[0041] In the present invention, Group III Elements in the Group IIInitride compound semiconductors may be partially substituted by boron(B) or thallium (Tl), and nitrogen atoms may be partially substituted byphosphorus (P), arsenic (As), antimony (Sb), or bismuth (Bi). When thesesemiconductors are used in light-emitting devices, 2-component or3-component Group III nitride semiconductors are preferred.

[0042] In the first and second embodiments of the invention, thebackside of the substrate is formed from a metallic negative electrode5B or 18B. The metal electrode may further be coated with another layer.For example, in a light-emitting diode, a light-reflecting layer may beformed on the backside metal layer in order to enhance light extractionefficiency. Examples of preferred metals for forming thelight-reflecting layer include Al, In, Cu, Ag, Pt, Ir, Pd, Rh, W, Mo,Ti, and Ni. These metals may be used singularly or in combination of twoor more species, for instance, in the form of an alloy. The metalliclight-reflecting layer having the same construction as above may bedisposed on the positive electrode side; for example, in alight-emitting diode.

[0043] In the above embodiments, the Al_(0.07)Ga_(0.93)N conductivesubstrate was obtained without doping. However, an n-typeAl_(x)Ga_(1−x)N substrate having a controlled conductivity may be formedby using silane (SiH₄) to dope silicon (Si), and a Group III nitridecompound semiconductor device may be formed thereon. In the embodimentsdescribed above, the compositional proportion of aluminum contained inthe substrate 1 or 11; the cladding layer 2, 4, 12, or 16; the guidelayer 13 or 15; or the barrier layer in the MQW represents a typicalexample. Thus, any material that satisfies the formula Al_(x)Ga_(1−x)N(0≦x≦1) may be employed. In such a case, the proportion ‘x’ may bevaried in accordingly in the components semiconductor layers.

[0044] In addition, the light-emitting devices of the above embodimentswere fabricated such that electricity passes through the Al_(x)Ga_(1−x)Nsubstrate. Alternatively, both the positive electrode and the negativeelectrode may be disposed on the top surface of the various devicelayers. Since one of the important features of the invention isemployment of an Al_(x)Ga_(1−x)N substrate in which ‘x’ is approximatelyequal to ‘x’ in a Group III nitride compound semiconductor layercharacterized by having a high value of ‘x’, the positions for disposingelectrodes are not limited.

[0045] While the invention has been described in terms of a certainpreferred embodiment, other embodiments apparent to those of ordinaryskill in the are also within the scope of this invention. Accordingly,the scope of the invention is intended to be defined only by the claimsthat follow.

What is claimed is:
 1. A Group III nitride compound semiconductor devicecomprising a substrate and one or more Group III nitride compoundsemiconductor layers on a first surface or first and second surfaces ofthe substrate, wherein aluminum gallium nitride satisfying the formula(Al_(x)Ga_(1−x)N, 0<x<1) is employed as the substrate.
 2. A Group IIInitride compound semiconductor device according to claim 1, whereinamong the Group III nitride compound semiconductor layers stacked on thefirst surface or the first and second surfaces of the substrate, alllayers having a thickness of more than 10 nm are independently formed ofa compound represented Al_(x)Ga_(1−x)N, wherein 0≦x≦1.
 3. A Group IIInitride compound semiconductor device according to claim 1, wherein eachof the Group III nitride compound semiconductor layers stacked on thefirst surface or the first and second surfaces of the substrate isformed of a compound represented by Al_(x)Ga_(1−x)N, wherein 0≦x≦1.
 4. AGroup III nitride compound semiconductor device according to claim 1,wherein a first layer of the Group III nitride compound semiconductorlayers stacked on the first surface or the first and second surfaces ofthe substrate has a thickness of 1-20 μm.
 5. A Group III nitridecompound semiconductor device according to claim 1, wherein a firstlayer of the Group III nitride compound semiconductor layers stacked onthe first surface or first and second surfaces of the substrate has athickness of 2-10 μm.
 6. A Group III nitride compound semiconductordevice according to claim 2, wherein each of the Group III nitridecompound semiconductor layers stacked on the first surface or the firstand second surfaces of the substrate is formed of a compound representedby Al_(x)Ga_(1−x)N, wherein 0≦x≦1.
 7. A Group III nitride compoundsemiconductor device according to claim 2, wherein a first layer of theGroup III nitride compound semiconductor layers stacked on the firstsurface or the first and second surfaces of the substrate has athickness of 1-20 μm.